1. Field of the Invention
The present invention relates to an air-operated valve with a piston slidable in a cylinder by operation air to activate a valve section.
2. Description of Related Art
As an air-operated valve arranged to activate a valve section by sliding a piston in a cylinder by operation air, there is for example an air-operated valve 1100 as shown in FIG. 31.
The air-operated valve 1100 includes, in outer shape or appearance, an actuator section 1110 attached to a body 1101, and further a hand-operated mechanism 1120 attached to the actuator section 1110 for forcibly actuating the actuator section 1110.
The body 1101 is formed with a valve seat 1104 between a primary-side passage 1102 and a secondary-side passage 1103. A diaphragm 1105 is placed with its outer edge clasped between the body 1101 and a holder 1106 while an adaptor 1107 is threadedly engaged in the body 1101 with the holder 1106 intervening therebetween. A stem 1108 is slidably mounted in the holder 1106 and held in contact with a back-pressure side of the diaphragm 1105. The actuator section 1110, which is threadedly engaged in the adaptor 1107 so that a center rod 1118 abuts on the stem 1108, is coupled to the body 1101.
Parts or components of the actuator section 1110, except for O-rings, are made of rigid metal such as stainless steel for ensuring pressure resistance to operation air. The actuator section 1110 includes a hollow base 1111 and a cap 1112 which are threadably connected to each other to constitute a cylinder. Those base 1111 and cap 1112 hold a partition plate 1113 therebetween, thereby defining a first piston chamber 1114 and a second piston chamber 1115 partitioned by the partition plate 1113. In the first and second piston chambers 1114 and 1115, first and second pistons 1116 and 1117 are slidably mounted respectively, forming hermetically divided pressure chambers 1114a, 1115a and backpressure chambers 1114b, 1115b. 
The center rod 1118 is disposed passing through the first piston 1116, the partition plate 1113, and the second piston 1117 and fixed to the first and second pistons 1116, 1117 respectively. In the backpressure chamber 1114b of the first piston chamber 1114, a compression spring 1119 is set in compressed form so that its resilient (or elastic) force acts on the stem 1108 through the first piston 1116 and the center rod 1118 to bring the diaphragm 1105 into contact with the valve seat 1104.
The center rod 1118 has a main flow passage 1118a bored therein extending from an upper surface to a middle point along a central axis of the center rod 1118, and branch passages 1118b, 1118c formed extending perpendicular to the main flow passage 1118a. The branch passages 1118b, 1118c are so formed as to communicate with the pressure chambers 1114a, 1115a respectively. An upper end of the center rod 1118 is placed in an air supply and exhaust passage 1112b formed in the cap 1112 to allow operation air to be supplied to the pressure chambers 1114a, 1115a via the flow passages 1118a, 1118b, and 1118c of the center rod 1118 or to be discharged from the pressure chambers 1114a, 1115a. On the other hand, the cap 1112 is formed with a first breathing hole 1112a communicating with the backpressure chamber 1114b. The base 1111 is formed with a second breathing hole 1111a communicating with the backpressure chamber 1115b. Accordingly, in the actuator section 1110, the center rod 1118 is caused to move upward and downward in the figure in accordance with balance between the resilient force of the compression spring 1119 and the pressure of the operation air acting on the pressure chambers 1114a, 1115a. 
In the air-operated valve 1100 having the above configuration, while no operation air is supplied to the air supply and exhaust passage 1112b, the resilient force of the compression spring 1119 acts on the diaphragm 1105 through the first piston 1116, center rod 1118, and stem 1108 to hold the diaphragm 1105 in contact with the valve seat 1104. In this case, a control fluid supplied to the primary-side passage 1102 is blocked from flowing in the secondary-side passage 1103 via the valve seat 1104.
When the operation air is supplied to the air supply and exhaust passage 1112b and the inner pressures of the pressure chambers 1114a, 1115a exceed the resilient force of the compression spring 1119, the center rod 1118 is moved upward in the figure away from the stem 1108. Accordingly, the diaphragm 1105 is not pressed toward the valve seat and thus moves away from the valve seat 1104 by its own reaction force. When the control fluid is then supplied to the primary-side passage 1102, the control fluid is permitted to flow from the primary-side passage 1102 to the secondary-side passage 1103 via the valve seat 1104.
Thereafter, when the operation air is discharged from the pressure chambers 1114a, 1115b respectively through the air supply and exhaust passage 1112b, the inner pressures of the pressure chambers 1114a, 1115b is reduced below the resilient force of the compression spring 1119, allowing the center rod 1118 to move downward. The center rod 1118 presses the diaphragm 1105 through the stem 1108 toward the valve seat, thus bringing the diaphragm 1105 into contact with the valve seat 1104. This interrupts the flow passage, blocking the control fluid from flowing from the primary-side passage 1102 to the secondary-side passage 1103 via the valve seat 1104.
The above conventional valve is disclosed in for example JP2005-214231A.